1. Field of the Invention
This invention relates generally to circuits for processing switch actuations to electronic circuitry, and in particular to an improved passive input circuit that processes closure of a grounded mechanical input switch to a CMOS input. The invention is especially useful in D.C. electrical systems of automotive vehicles.
2. Background Information
Automotive vehicles have various electronic modules that perform various functions associated with vehicle operations. Possible types of inputs to such modules include inputs from mechanical switches. Mechanical switches are useful in automotive vehicles for various reasons, including for example, their suitability for the diverse environmental conditions which vehicles may encounter during their useful lives, their economical cost, and their general reliability. A grounding type mechanical switch may be useful in any of various applications in an automotive vehicle.
Because of the nature of certain grounding type mechanical switches and the nature of certain automotive vehicle electrical systems, the closure of such a switch may not always cause a true ground, i.e. zero volts, at an input of an electronic module that monitors, or reads, the switch. Such an input may include a ground offset, meaning that the actual voltage input applied to a module upon switch closure is close to, but not precisely, zero volts. The closed switch may also exhibit a certain series impedance in the input to the module to which it is electrically connected. Moreover, an electrical charging system of an automotive vehicle may deliver an actual power supply voltage that varies considerably from nominal. Voltage extremes of a nominal +12 volt D.C. electric power supply in an automotive vehicle may range from a few volts above ground to voltages well above nominal, such as from +6 volts D.C. to +18 volts D.C. for example.
For processing closures of such switches in automotive vehicles, associated modules may include embedded microcomputers. Such microcomputers may have CMOS type input structures. Accordingly, it is known to process the closure of a grounded switch into a CMOS input. A known processing circuit for level shifting a ground side switch into a CMOS input comprises a diode and three resistors. In a positive polarity electrical system, the anode of the diode is connected to the positive supply voltage and the cathode of the diode is connected to a terminal of a first of the three resistors. The second and third resistors are connected in series between the other terminal of the first resistor and ground. The output terminal of the mechanical switch is connected to the junction of the first and second resistors while the switch input terminal is connected to the electrical system ground. The junction of the second and third resistors is connected to the appropriate CMOS input of the module that monitors the condition of the switch.
The diode provides reserve voltage protection by preventing any negative transients that may occur on the voltage supply line from reaching the CMOS input. The first resistor provides a potential that the mechanical switch can pull against, and it also provides a certain switch wetting current that may aid in keeping the switch contacts clean. The second resistor limits the electric current that can be injected into the CMOS input due to higher than nominal voltages (D.C. or transient) on the supply voltage line. The third resistor completes a voltage divider that, when the switch is closed, keeps a positive ground offset voltage from being improperly interpreted, or improperly read, as a logic "1" signal. One advantage of the known processing circuit that has just been described is that it comprises relatively few circuit components.
The present invention arises from the inventor's recognition of certain limitations of that circuit which, under certain conditions, may cause the actual switch condition to be misinterpreted, or misread, either as a logic "1" signal when the correct signal should be a logic "0" signal, or as a logic "0" signal when the correct signal should be a logic "1" signal.
The inventor believes that such misreading may be due to any one or more of various causes including: greater than 5% variance in resistance values of nominal 5% tolerance resistors because of factors like component aging and thermal effects; an inability of a CMOS input to determine the logic value of an input over a certain intermediate portion of an input voltage range (for example an intermediate range between +1 volt and +3.5 volts in the case of a module that uses a +5 volt supply voltage); disparity between actual values of ground voltage at the switch and ground voltage at the module; significant switch impedance when closed; departure of actual supply voltage from nominal (i.e. +6 volts to +18 volts).
Factors other than those just mentioned (switch, wiring harness, and PWB leakage, for example) may also play roles, but their impact is believed less significant. The known processing circuit described above is believed suitable for supply voltages ranging from +10 volts to +16 volts, in spite of the adverse influence of the factors previously mentioned.
Because it is possible that the supply voltage in an automotive vehicle may vary over a more extensive range than between +10 volts to +16 volts, further improvements in switch closure processing circuits is seen to be desirable. While it is possible that such improvements may be accomplished by active, as distinguished from passive, processing circuits, active processing circuits generally add complexity and cost. Accordingly, it is believed most desirable to achieve a solution that retains the use of a passive processing circuit.
A preliminary novelty search in connection with this invention developed the following U.S. Pat. Nos.: 4,420,669; 4,532,432; 5,184,026; 5,754,890; and 5,783,875; as representative of the state of the art.